Beneficiation of chromium ore to reduce the iron content



United States Patent 3,508,906 BENEFICIATION 0F CHROMIUM ORE TO REDUCE THE IRON CONTENT Henry K. Bruner, Cambridge, Ohio, assignor to Foote Mineral Company, Exton, Pa., a corporation of Pennsylvania No Drawing. Filed Aug. 30, 1967, Ser. No. 664,288 Int. Cl. C22b 1/100, 3/100 US. Cl. 75-1 8 Claims ABSTRACT OF THE DISCLOSURE A method of beneficiating chromium ores in which pulverized chromium ore is heated with carbon under a vacuum to reduce iron oxides. Thereafter, ore is leached with acid to form soluble iron salts which are washed away leaving a product with a high Cr/ Fe ratio.

This invention relates to a method of beneficiating chrominum ore. More particularly, it relates to a method of removing iron from chrome ores to produce chrome ores having high chromium to iron ratios Which are suitable for the production of chrome metal and high chromium ferrochromium.

The principal source of chromium is chromite ore which consists of chromium oxide, iron oxide, magnesia and alumina, plus minor percentages of silica and other incidental impurities. Most commercial chromite ores contain from about 40% to about 55% Cr O and have a Cr/ Fe ratio (all ratios used herein are based on the weight percent of the Cr and Fe) ranging from about 1.5 to 4.0 or in some cases slightly higher. Good metallurgical grade chromite ores contain a minimum of 48% Cr O and have a Cr/Fe ratio of 3 or higher. These ores are suitable for the manufacture of commercial grades of ferrochromium alloys such as low carbon ferrochromium, high carbon ferrochromium and ferrochrome-silicon, which are suitable for adding chromium to steel and many high temperature alloys Which contain a substantial percentage of iron.

There is an increasing need for chromium in alloys which contain little or no iron. It is not possible to use ferrochromium produced from chromite ore as an additive to such alloys because of the large percentage of iron which would be added with the chromium. Therefore, it is usual to add chromium to such alloys in the form of chromium metal which is produced either by electrolytic means or 'by aluminum reduction of chromium oxide. It is much more costly to use chromium as chromium metal produced by either such means than it is to use chromium in the form of ferrochromiu-m.

A beneficiated chromite ore with a high Cr/Fe ratio could be used to produce a high chromium ferrochromium, approaching the purity of chromium metal, by conventional smelting procedures. A high chromium ferrochromium produced in this manner would contain substantially less iron than regular ferrochromium and would fill the existing gap between ferrochromium and chromium metal.

Many methods have been proposed for removing iron from chrome ores, including reducing the iron oxide with carbon at atmospheric pressure, treating the ore with chlorine and carbon or carbon monoxide and treating the ore with anhydrous calcium chloride in a vacuum. These processes yield a chrome ore having improved Cr/Fe ratios.

I have invented a new method of beneficiating chrome ores which results in chrome ores having substantially higher Cr/ Fe ratios than are achieved by proceses now in use. In addition, my process is both economical and efficient. Briefly stated, it comprises heating pulverized 3,508,906 Patented Apr. 28, 1970 chrome ore and carbon and preferably also magnesia in a vacuum furnace to preferentially reduce the iron oxide followed by acid leaching of the product to remove the iron.

My method of beneficiating chrome ore is applicable to any chrome ore containing iron. The chrome ore is mixed with a carbon reducer such as coal, coke, or charcoal. The amount of carbonaceous reducer may be between and 250 percent of the stoichiometric amount needed to reduce the iron in the ore; however, I prefer to use about to percent of the stoichiometric amount. Although more or less carbon may be used, excessive amounts will cause chromium losses due to the reduction of chromium, and inadequate amounts of reducer will prevent high Cr/ Fe ratios because of the lack of iron reduction.

The ore and carbonaceous reducer are preferably pulverized in order to give a large surface contact between the iron and the reducer. The particles should be ground to less than about 48 mesh and preferably to less than about 325 mesh.

Since the furnace will be under vacum, it is desirable to provide some means to prevent the charge materials from being sucked into the vacuum blower. I have found that this is best accomplished by aggregating the charge materials in the form of pellets or briquettes. Success has been obtained using briquettes 1%" by 1 /2" by 1".

I prefer to add some magnesia to the furnace charge. I have found that the Cr/ Fe ratio of the product may be further increased by adding magnesia in an amount equal to about 5 to 45 percent of the chrome ore to the furnace charge. I prefer that the magnesia added be about 20 to 30 percent of the chrome ore in the charge. Magnesia apparently replaces FeO in chromite and thereby facilitates the reduction of FeO. Although more or less magnesia may be used, excessive amounts of magnesia dilute the final product and increase acid consumption since some magnesia is removed during leaching.

The ore and the reducer are placed in a vacuum furnace and heated to about 1900 F. to 3000" F. at a pressure of less than 10,000 microns (10 mm. of Hg). I prefer to use temperatures in the range of 2200 F. to 2500 F. The reaction proceeds at uneconomically low rates at lower temperatures, while at higher temperatures excessive amounts of chromium may be reduced. The use of carbon reducer enables the reaction to take place in the solid state at relatively low temperatures under vacum. If non-carbon reducers were used, the reaction would have to take place in the molten state.

During the reaction, the gases produced in the furnace (principally carbon monoxide) are continuously removed and a vacuum maintained between about 200 to 10,000 microns. The vacuum permits the reaction to proceed at a lower temperature, allowing the ore to remain in a solid state.

Completion of the reaction is indicated by a rapid drop in pressure due to a decrease in the amount of gas produced by the reaction. Preferably the pressure should be reduced to less than about 200 microns. Upon completion of the reaction, the charge materials are removed from the furnace and are preferably ground again.

The product is then leached with acid to remove the reduced iron by forming soluble salts. Any suitable acid may be used, but sulfuric acid is preferred because it is inexpensive and reacts rapidly. Although the amount of acid used will depend upon the amount of iron which has been reduced. I have found that about 1 to 3 lbs. of sulfuric acid in 15% solution per lb. of processed ore is generally adequate, and about 1.5 lbs. of sulfuric acid in 15% solution is preferred.

The treatment in the furnace reduces most of the iron oxide in the ore to free iron. A small amount of the chromium oxide is also reduced to the metallic state.

The following non-limiting examples of the beneficiation of chromium ore illustrate the use of the methods of the invention.

EXAMPLE I A charge was prepared by grinding 130 lbs. magnesia, 30.75 lbs. Cabot coke and 500 lbs. Transvaal chrome ore is less than 325 mesh in a ball mill. The Transvaal chrome ore had a CR/Fe ratio of 1.55 and the following composition:

Percent CI203 Fe 19.08 SiO 4.02

The ground mix was briquetted and 250 lbs. of briquettes were placed in a vacuum induction furnace containing a graphite crucible. The briquettes were heated to 2300 F. at a pressure of 9800 microns for 5.5 hours when completion of the reaction was indicated by a pressure drop to 680 microns. After cooling, the briquettes were removed from the furnace and ground to less than 200 mesh in a ball mill followed by leaching in a 15 percent sulfuric acid solution. The processed ore contained 47.18 percent Cr O and 0.90 percent Fe and had a Cr/Fe ratio of 35.9.

EXAMPLE II A second charge was prepared by mixing 500 lbs. Rhodesian chromium ore containing 53.47 percent Cr O and 13.96 percent Fe and having a Cr/Fe ratio of 2.62 with 130 lbs. magnesia and 23.4 lbs. Cabot coke and grinding the mix in a ball mill to less than 325 mesh. The mix was formed into briquettes, 250 lbs. of which were placed into a vacuum induction furnace and heated to 2320 F. at 7500 microns for 5.5 hours when the pressure dropped to 70 microns. The briquettes were then ground to less than 200 mesh and leached in 15 percent sulfuric acid. The processed ore contained 56.87 percent Cr O and 1.34 percent Fe and had a Cr/Fe ratio of 29.0.

From the foregoing, it may be seen that this invention provides a method of removing iron from chromium ore by heating the ore with carbon under a vacuum followed by acid leaching. Still higher Cr/Fe ratios are obtained by adding magnesia to the ore.

While I have described my invention in terms of the present preferred embodiment, it may be otherwise embodied within the scope of the following claims.

I claim:

1. A method of beneficiating chrome ore by removing iron therefrom comprising:

(1) mixing chrome ore containing iron oxide with a carbonaceous reducer, the amount of the reducer being between 90 and 250 percent of the stoichiometric amount required to reduce the iron oxide in the ore (2) heating the mixture in a vacuum furnace to a temperature of between 1900 F. and 3000 F. under a vacuum until the evolution of gas is substantially decreased (3) removing the mixture from the furnace and (4) leaching the mixture with acid to remove metallic iron.

2. The method set forth in claim 1 in which magnesia is mixed with the chrome ore and carbonaceous reducer 4 in Step (1), the amount of magnesia being about 5 to 45 percent by weight of the chrome ore weight.

3. A method of beneficiating chrome ore by removing iron therefrom comprising:

(1) mixing chrome ore containing iron oxide with a carbonaceous reducer, the amount of reducer being between and 250 percent of the stoichiometric amount required to reduce the iron oxide in the ore (2) reducing the particle size of the mixture to less than about 48 mesh (3) forming agglomerates of the mixture (4) heating the agglomerates in a vacuum furnace to a temperature between about 1900 F. and 3000 F. under a vacuuum of less than 10,000 microns (5) removing the processed ore from the furnace when the pressure in the furnace drops rapidly indicating a substantial decrease in gas evolution (6) leaching the processed ore with acid to remove metallic iron.

4. The method set forth in claim 3 in which magnesia is mixed wtih the chrome ore and carbonaceous reducer in Step (1), the amount of magnesia being about 5 to 45 percent by weight of the chrome ore weight.

5. The method set forth in claim 4 in which the magnesia constitutes about 20 to 30 percent by weight of the chrome ore weight.

6. The method set forth in claim 3 in which the carbonaceous reducer constitutes about to percent of the stoichiometr-ic amount required to reduce the iron oxide in the ore.

7. The method set forth in claim 3 in which the temperature in the vacuum furnace is maintained between 2200 F. and 2500 F.

8. A method of beneficiating chrome ore by removing iron therefrom comprising:

(1) mixing chrome ore containing iron oxide with (a) a carbonaceous reducer, the amount of the reducer being between 125 and 175 percent of the amount required to reduce the iron oxide in the ore, and

(b) magnesia, the amount of magnesia being about 20 to 30 percent by weight of the weight of the chrome ore (2) reducing the particle size of the mixture to less than about 325 mesh (3) forming agglomerates of the mixture (4) heating the agglomerates in a vacuum furnace to a temperature between about 2200" F. and 2500 F. under a vacuum of less than 10,000 microns (5) removing the product from the furnace when the pressure in the furnace drops rapidly indicating a substantial decrease in gas evolution and (6) leaching the product with acid to remove metallic iron.

References Cited UNITED STATES PATENTS 52,120 1/ 1866 Absterdam 756 2,256,536 9/1941 Udy 75-1 2,905,546 9/1959 Harris 751 OSCAR R. VERTIZ, Primary Examiner G. T. OZAKI, Assistant Examiner US. Cl. X.R. 75-101; 23-145 

